Question: "Did Constantine change the Sabbath from Saturday to Sunday?"
Answer: It is not secret knowledge that in 321, Constantine decreed, "On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed" (Codex Justinianus lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol. 3, p. 380, note 1). Constantine seems to have made this change himself and not through the papacy, since the "papacy" had not really come in to being at that time. The papacy grew gradually out of the office of Bishop and for many years this was centered in Rome. In any case, it should be noted that in doing this, Constantine is not changing the Sabbath; he is merely making Sunday the official day of rest for the Roman Empire. His motivation was probably not born out of hatred for the Jews (it's hard to say for sure why Constantine or any historical figure did what they did) but out of adoption of the practice that Christians had already been doing for nearly two and a half centuries.
http://www.gotquestions.org/Constantine-Sabbath.html